1. Field of the Invention
This invention relates to a thermosettable adhesive or sealant plastisol dispersion composition which, on heating at or above the fluxing temperature, rapidly provides handling strength and can be crosslinked to give a thermoset seal or bond by heat or UV radiation.
The invention also relates to a process for forming a crosslinked, bond or seal.
2. Description of the Prior Art
Conventional plastisol compositions are made up of finely divided thermoplastic resin particles in a liquid, non-volatile plasticizer in which the resin is insoluble at room temperature. On elevating the temperature of the composition, the resin particles are substantially completely plasticized by the plasticizer producing a homogeneous solid solution which is a rubbery, thermoplastic gel.
The only widely used resin in plastisol compositions has been polyvinyl chloride because of certain inherent characteristics. Vinyl chloride resins produced either by bulk, suspension or aqueous emulsion polymerization or mixtures thereof have been found to be particularly advantageous in such compositions. Vinyl chloride resins produced by aqueous emulsion polymerization, because of their excellent stir-in properties which permit easy dispersion in a liquid plasticizer up to a high concentration of the resin at room temperature to form paste like mixtures, have been found useful in many applications.
However, vinyl chloride resins whether produced by bulk, suspension or by emulsion polymerization, have certain drawbacks. Vinyl chloride resin decomposes upon heating in processing, releases corrosive hydrogen chloride and, ultimately, results in the discoloration and deterioration of fabricated polymer properties. A problem recently noted in regard to polyvinyl chloride is the possible carcinomatous effect these polymers may have on the human body due to possibly residual vinyl chloride monomer being present in the polymer. In light of the above, there has been a recent search for materials which can be substituted for polyvinyl chloride in various end uses.
The conventional plastisols after plasticization normally have low heat resistance resulting from the plasticization which reduces the heat deflection temperature of the polymers. To improve the heat resistance, reactive plasticizers such as diacrylates (Dainippon Ink and Chemical JP 80 52,335) and epoxy resins (Dunlop, JP 81 00841) were mixed with conventional plasticizers to provide crosslinks after curing the fluxed plastisol.
In general, a thermoset is based on two reactive ingredients separated physically as two packages. This separation leads to the problems of non-uniform mixing and inaccurate metering in application. Furthermore, the materials, after mixing, have to be used in a very short period to prevent gellation caused by the crosslinking reaction. This creates difficulties such as pot life, quality control and the economic concerns of a two-package thermoset system.
To prepare a one package thermoset, technology such as chemical blocking of the reactive group was developed. Unfortunately, the available one package systems suffer from short shelf life and reduced curing speed. A solution to this problem is to prepare reactive plastisols which rapidly turn to a flexible or rubbery solid upon heating at a temperature above the fluxing point to provide handling mechanical strength and cure to a wholly crosslinked thermoset at the same or more elevated temperature after fluxing. In these materials the plasticizer for the thermoplast contains functional groups which in the presence of latent thermal curing agents allow the plasticizer to cure on heating. One example of such a system is set out in U.S. Pat. No. 4,020,966. Therein a liquid epoxide plasticizer and a resin component comprising a copolymer of a normal alpha-olefin and maleic anhydride are admixed at room temperature and heated to an elevated temperature, e. g., 200.degree. C., resulting in a fluxed, thermoset product.